The present invention is directed to a method and apparatus for forming a weld in a member moving at high speed relative to a high energy density welding device such as a laser. More particularly, the invention relates to the formation of a continuous weld along the length of a member moving at high speed relative to a pulsed high energy density welding device such as a pulsed laser wherein the continuous weld is made of a plurality of overlapping spot welds formed by pulses of energy having an average energy or power density of at least 10.sup.6 watts/inch.sup.2. The method and apparatus of the invention may be used to form high quality welds in materials such as thin sheet metal for making can bodies and the like at speeds at least comparable to other joining techniques.
The speed at which high quality autogenous welds can be made using a high power laser has been a subject of considerable interest in recent years, especially as it relates to the welding of thin sheet material. The June, 1981 Welding Journal article entitled "The Laser Welding of Steels Used In Can Making" by J. Mazumder and W. M. Steen reports on an investigation undertaken to establish the ability of a laser to weld the metals used in can making. The operating and metallurgical results of laser welding 0.2 mm (0.008 in.) thick tin plate and tin-free 0.4 mm thick steel with a 2 kW continuous wave (CW) CO.sub.2 laser were recorded during the investigation. Weld speeds attained with lap welding of the 0.2 mm material were 7-8 meters per minute (m/min.) and 10-20 m/min. for bead-on-plate welds in 0.2 mm material. Mazumder and Steen note that these speeds are low compared to present can making speeds employing other joining techniques such as lock seam-soldering and electrical resistance welding. However, the authors state that higher speeds can be achieved with laser welding by augmenting the laser with an electric arc. It is reported that speeds of approximately 60 m/min. were attained with bead-on-plate welds on 0.2 mm thick tin plate using a laser on one side of the sheet and an electrical arc on the opposite side of the sheet.
U.S. Pat. No. 4,315,132 to Saurin et al. discloses a process and apparatus for welding tubular bodies, particularly for preserved food cans, wherein welding speeds on the order of 20 m/min. in sheet material 0.2 to 0.3 m/min. thick are reported using a continuous emission 1 kW laser. While Saurin et al. suggest that the speed may be substantially increased with a more powerful laser of 2 kW for example, applicant has found that high quality CW laser welds cannot be consistently obtained at speeds above about 22 m/min. with the Saurin et al. method regardless of the increase in laser power. That is, laser welds formed by a continuous laser beam in materials moving at speeds above approximately 22 m/min. are prone to surface irregularities and undercutting which are unacceptable for can making and most other applications.
Problems associated with high speed welding have been variously referred to in the art as surface rippling, humping or slubbing. T. R. Anthony and H. E. Cline present a theoretical discussion of surface rippling during laser welding in their article "Surface rippling induced by surface-tension gradients during laser surface melting and alloying" in the Journal of Applied Physics, Volume 48, No. 9, September, 1977. The authors indicate that surface rippling is induced during laser welding by surface-tension gradients. They also state that a high power laser may itself cause surface rippling through other mechanisms such as vapor-pressure depression of the melt surface. The phenomenon of undercutting and humping associated with high speed welding is also discussed by B. J. Bradstreet in his July, 1968, Welding Journal article entitled "Effect of Surface Tension and Metal Flow on Weld Bead Formation".
Regardless of the theoretical explanation of the problems associated with high speed welding, the practical effect of such phenomena is to prevent the attainment of consistently high quality laser welds at high speeds, such as 22 m/min. or more. For example, FIGS. 22A-22D of the drawings illustrate bead-on-plate CW laser welds made at a speed of 30 m/min. Weld irregularities or defects 98, 120 and 121 exist at periodic intervals along the length of the weld. During welding molten metal is dragged along the weld joint and periodically dumped at locations 98 leaving channeling 120 beneath the deposits, while weld joint portions between the deposits 98 are subject to undercutting and incomplete weld nugget formation with thru slots of the type shown at 121, for example, the causes of these weld irregularities are not fully understood but are believed to be related to the surface-tension gradients on the molten metal from temperature and pressure gradients in and around the weld puddle during laser welding as referred to above. Increasing the laser power during welding does not eliminate the problem.
The suggestion that higher laser power necessarily results in higher welding speeds is also disputed by Steen in his U.S. Pat. No. 4,167,662 wherein it is indicated that welding speeds using 15-20 kW lasers have resulted in performance well below those suggested by linear extrapolation from low power work. Possible reasons advanced for this are (1) optical design, (2) masking the workpiece by the laser induced plasma, or (3) some other reason. The patentee states that in any case, the low performance is primarily due to lack of energy reaching the workpiece. Steen suggests that the way ahead for laser processes is not by way of higher laser powers. Rather, Steen's invention is directed to electric arc argumented laser welding to increase welding speed.
A continuous seam weldment is established between two strips of sheet material while the strips are moving by forming a converging Vee geometry between the moving strips, applying a pressure at the point of convergence by pressure rolls and focusing a laser beam into the converging Vee according to U.S. Pat. No. 4,185,185 to Adlam. High welding speeds are reported with this method. However, the use of pressure rolls and continuous strips of material limit the applicability of this type of process. The process is also limited to the production of lap welds.
An object of the present invention is to provide a method and apparatus for forming a continuous weld at high speed which avoids the aforementioned disadvantages and limitations associated with the known joining methods and apparatus. More specifically, an object of the invention is to provide a method and apparatus for forming a continuous weld in a member moving at a high speed, typically 22 m/min. or more, relative to a high energy density welding means such as a laser whereby a weld of high quality can be consistently obtained with the problems of surface rippling, humping and slubbing being avoided.
An additional object of the invention is to provide a method and apparatus for forming a continuous laser weld at high speed in a thin material for manufacturing cans, for example, which do not require the use of an electrical arc to augment the power of the laser or the use of pressure rollers and which method and apparatus are not limited in their applicability to continuous strips of material or to lap type joint configurations but may be used to laser weld discrete articles, such as individual can bodies, regardless of joint configuration.
A further object of the invention is to provide a method and apparatus for high speed welding of coated materials, particularly metal sheet material coated with nonmetallic material.
Swiss Pat. No. 593,757 to Feller discloses a process for welding coated sheet or leaf metal parts together using a CO.sub.2 laser wherein the coating is not removed prior to welding. Feller states that the coating is burned or vaporized during the welding. The patent to Feller does not teach or suggest a solution to the aforementioned problems of high speed laser welding or, more particularly, the problems of high speed laser welding of coated materials. Applicant's experience indicates that the continuous laser welding process of Feller is suitable only for the low speed welding of certain types of plain or coated materials. High speed laser welding of coated materials with the Feller method is problematical for the reasons indicated above with respect to high speed welding in general and, moreover, because at high speeds the problems of loss of welding energy at the work due to reflection by the coating or vaporization thereof may be aggravated. Also, the coating material can become distributed in the weld puddle which may be detrimental to the metallurgical properties of the weld causing weld porosity and/or weld cracking. These problems make some coated materials impossible to laser weld, especially at high speeds. In addition, with a continuous process of the type suggested by Feller, the contaminants may be dragged along the weld and concentrated periodically in the weld puddle causing irregular welds that are weak or structurally unsound and cosmetically unacceptable. For example, porosity can be induced in the contaminated weld areas to produce a defective weld.
Thus, another object of the present invention is to provide a method and apparatus for laser welding coated materials at high speeds and with high weld quality whereby the aforementiond problems associated with the prior art methods are avoided or minimized.
An additional object of the invention is to provide a method and apparatus for forming a continuous laser weld at high speed in a member such that the energy for welding from the laser per unit length of the member can be accurately controlled and varied in a predetermined manner so as to optimize the weld quality for a given material.
These and other objects are attained according to the invention by providing a method of forming a continuous weld at high speed comprising the steps of moving a member to be welded at high speed relative to a pulsed high energy density welding means such as a laser, forming a continuous weld along the member with a plurality of overlapping welds formed by pulses of energy from the pulsed high energy density welding means, the pulses having an average energy density of at least 10.sup.6 watts/inch.sup.2, detecting the movement of the moving member in the vicinity of the welding means and controlling the pulsed high energy density welding means during the welding in response to the detected movement of the moving member so that the energy for welding from the pulses per unit length of the member is accurately controlled.
While pulsed laser welding, per se, at low welding speeds is known, see the December, 1965 Welding Journal article by J. E. Anderson and J. E. Jackson entitled "Theory and Application of Pulsed Laser Welding" and also more recently U.S. Pat. No. 4,152,575 to Banas, for example, it has been discovered that the problem of surface irregularities and weld defects which limit the speed at which high quality welds can be made in thin materials as discussed above can be overcome by welding at high speed with intermittent pulses of laser energy. Each of the pulses has a duration of only microseconds and forms a discrete spot weld. The spot welds are formed so as to overlap and form a continuous weld. The surface rippling, humping and slubbing problems associated with high speed CW laser welding are avoided or substantially overcome by this method. That is, with the method of the invention there is no continuous molten weld puddle to be swept along by surface-tension gradients but rather a series of discrete puddles which quickly solidify before the surface-tension gradients or vapor pressure depression of the melt can produce irregularities. Therefore, the method and apparatus of the invention make it possible to produce laser welds of high quality in materials at speeds heretofore unattainable without the use of special provisions such as electrical arc argumentation of the laser power or the use of pressure rollers in combination with the laser beam.
The high speed laser welding of materials having coatings and contaminants thereon is also facilitated with the method and apparatus of the invention by pulsing the laser beam to form a series of overlapping spot welds. The pulses of laser energy vaporize the coating during welding and reduce the weld puddle contamination. For this purpose, each pulse of laser energy preferably includes a relatively high peak of laser power at the beginning thereof which vaporizes the coatings and contaminants and aids absorption of the welding energy by the member. Because the continuous weld is formed by a series of overlapping spot welds, contaminants from the coating have less time to enter the weld puddle and are not swept along and periodically concentrated as with a weld formed by a continous laser beam. Instead, any weld contaminants which are not vaporized are distributed along the weld in a concentration level and/or frequency which is generally not high enough to cause the previously mentioned metallurgical problems associated with welding such materials. A weld formed at high speed according to the invention also has a very even, uniform appearance.
Further, according to the method of the invention, the high speed movement of the member to be welded is detected in the vicinity of the pulsed high energy density welding means and the pulsed high energy density welding means is controlled during welding in response to the detected movement so that the energy for welding from the pulses per unit length of the member is accurately controlled. More specifically, by controlling the actual pulsing of the welding means such as a laser in response to the detected movement of the member, the pitch of the plurality of overlapping welds can be made substantially constant even with changes in the speed of movement of the member.
According to a disclosed, preferred embodiment of the invention, the method step of detecting the movement of the moving member includes detecting the leading edge of the moving member upstream of a pulsed laser welding means and continuously detecting the position of the moving member as it moves past the pulsed laser welding means. This permits the accurate initiation of laser welding with respect to the moving member and the accurate control of the energy for welding along the length thereof.
The trailing edge of the moving member is also detected upstream of the pulsed laser welding means according to the method. This, together with the continuous detection of the position of the member as it moves past the pulsed laser welding means, enable the pulsed laser welding means to be controlled so as to terminate welding at a precise, predetermined location along the moving member with respect to the trailing edge thereof.
Further, in the disclosed embodiment the step of controlling the pulsed laser welding means of the invention includes varying or adjusting the power of the laser pulses in a predetermined manner in response to the detected movement of the moving member. Thus, the step of controlling the pulsed laser welding means may include initiating the welding of the member at a first relatively low laser power level and then increasing the laser power level to a second relatively high power level at a predetermined distance along the moving member. The power level may also be decreased from the second relatively high power level to a third relatively lower power level at a predetermined distance before the end of the weld. As discussed more fully hereinafter, the accurate control of the laser power along the length of a member being welded permits heat input related metallurgical welding problems, such as thermally induced metallurgical failure or tearing at the beginning or end of welds, to be minimized or avoided.
An apparatus of the invention for forming a continuous weld along a member at high speed with a plurality of overlapping welds formed by pulses of energy comprises a pulsed high energy density welding means, means for moving a member to be welded at high speed relative to the pulsed high energy density welding means, means for detecting the movement of the moving member in the vicinity of the welding means, and control means for controlling the pulsed welding means in response to the detected movement of the moving member so that the energy for welding from the pulses per unit length of the member is accurately controlled. In the disclosed, preferred embodiment the pulsed high energy density welding means is a pulsed laser welding means and the control means for controlling the pulsed laser welding means controls the pulsing of the pulsed laser welding means in response to the detected movement of the member thereby permitting the pitch of the plurality of overlapping welds along the member to be substantially constant even with changes in the speed of movement of the member.
The pulsed laser welding means preferably provides pulses of laser energy which each have a relatively high peak of laser power at the beginning thereof. This is particularly advantageous for welding thin sheet materials having coatings or contaminants thereon as the coatings are vaporized by the high power at the beginning of the pulses and thereafter the remaining energy of the pulses can be efficiently used to effect welding. By welding at high speed with discrete pulses, there is also less chance for a cloud of vaporized coating or contaminate to interfere with the transmission of light energy to the member being welded.
The means for detecting the movement of the moving member detects the leading edge of the moving member at or upstream of the pulsed laser welding means and continuously detects the position of the moving member as it moves past the pulsed laser welding means. According to the disclosed embodiment, the means for continuously detecting the position of the moving member includes an encoder connected to the means for moving the member. The output of the encoder is provided to the control means for controlling the pulsed laser welding means. In the illustrated embodiment, the control means includes a digital-to-analog converter which is driven by the output of the encoder to control the pulsed laser welding means in a predetermined manner with respect to the position or length of the member. The control means also includes means for firing or pulsing the laser at predetermined positions along the moving member whereby the pulse pitch can be maintained constant regardless of changes in the speed of movement of the member. This is accomplished by a circuit within the control means which counts the pulses from the encoder and fires or pulses the laser at predetermined pulse intervals which correspond to changes in position of the moving member.
In the illustrated embodiment, the members being welded are generally cylindrically shaped with longitudinally extending edges which are to be laser butt welded to form can bodies or the like. In the apparatus a Z-bar guide is provided for guiding the edges into abutting position for laser welding as the member is moved at high speed in the direction of the pulsed laser welding means. The leading and trailing edges of the moving members are detected by a detector such as a light source and light detector located on opposite sides of the moving member in the Z-bar guide upstream of the welding means. In another form of the invention these edges are detected at the welding site or pulsed laser welding means by a light detector which detects the light from the weld plasma, or the absence thereof, as the moving members move into and out of contact with the pulsing laser beam.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, one preferred embodiment in accordance with the present invention.